Tunable frequency selective circuits



lo R. F. @NEILL Erm. 2,232,864

TUNABLE FREQUENCY SELECTIVE CIRCUITS Filed Nov. 8, 1938 fz www TTONEY Patented Feb. 25, 1941 UNITED STATES PATENT OFFICE TUNABLE FREQUENCY SELECTIVE CIRCUITS of Delaware Application November 8, 1938, Serial No. 239,418 In Great Britain December 16, 1937 7 Claims.

This invention relates to tunable frequency selective circuits and though of -wide applicati-on is primarily intended for the radio frequency signal selection circuits of broadcast and other radio receivers.

The usual requirement of the tuned circuits of a broadcast radio receiver is that there shall be a constant predetermined band pass width and a constant 'predetermined band Ipass characterisl tic curve shape suited to the prevailing general broadcast conditions-at present those imposed `due to the adoption of a 9 kilocycle spacing between broadcast transmit-ters. Although these requirements can now be satised .to a high degree as regards the intermediate frequency circuits -of a superheterodyne receiver, vfor these circuits are, of course, of a fixed frequency which does not vary with tuning of the receiver as a whole, known variably tunable selection circuits such as are used in the radio frequency stages of "a receiver, fall far shout of the labove requirements and vary undesirably in their characteristics as the receiver tuning is varied. The present invention enables the above requirements to be substantially satisfied yas regards the variably tunable stages of a receiver, In known variably tunable signal 'pre-selection circuits of the b'an-d pass type both the Width of the band passed an-d the `degree of attenuation `alt a given frequency separation from the middle of the pass band, depend upon the tuning of the circuit, i. e., the frequency to which the middle of the pass band is adjusted, and lan important object of the present invention is to avoid this defect.

According to this Iinvention a varialbly tunable band pass selection iilter of the type comprising a plurality of coupled resonant circuits is characterized by the use of mixed lcoupling between said circuits and the provision of means, unicontrolled with the variable tuning reactances of the lfilter, for varying the total effective resistance (including all losses, such as condenser dielectric losses, inductance eddy current losses and so forth) in each resonant circuit as the filter tuning is varied so that the ratio of inductance (L) to resistance in each circuit varies accor-ding to `a predetermined law with variation in tuning. In general the requirement will be constant band width and constant selectivity land to satisfy this requirement the variation of resistance should be such that the ratio T 5 remains constant. In special cases, however, other requirements may arise and these may be satisfied :by suit-ably vchoosing the law of variation of Preferably the required variation in resistance is obtained by providing, in association with ea'ch resonant circuit, `a xed resistance in series with a small variable control condenser of predetermined law of variation, this ycondenser being gang controlled with the main tuning reactances of the lter. Where (the usual case) these reaotances are condensers, ythe control condensers may most simply lbe provided in the form of one 20 additional vane on each of the main tuning condensers.

The invention is illustrated in Ithe accompanying drawing, wherein Fig. 1 shows one form of band pass iilter in accord-ance with the invention; 25 Fig. 2 shows a modification of the iilter of Fig. 1; Fig 3 isl a constructional detail of :a portion of the filter disclosed in Fig. 2; and Fig. 4 shows the equivalent electrical circuit of Fig. 3.

Referring to Figure y1 the band pass lter is 30 constituted by a four termin-al two wire-network having one inpult terminal 2 directly connected to one output terminal `4 to constitute one wire, the other wire containing in series in the order stated, between the remaining input :terminal I and the remaining output terminal S, a rst inductan'ce I fl, a nrst resistance R'I, a second resistance R2, and a second inductance L2. The two in'ductances Ll, L2, are equal as also are the two resistances R1, R2. One of the two tuning reactances of the iilter is constituted by a Variable Icondenser' CI connected between the input terminals l, 2, and the other is constituted by a similar 'condenser G2 between the output terminals 3, 4, these two condensers being ganged as indicated. The coupling branch of the filter is of the mixed or compound coupling type and consists of an inductance ML and a condenser CC in series in the order stated between the 50 junction poinit of the two resistances RI, R2, and the wire which connects terminals 2, 4 The inductance ML may ibe a separate coil :but is preferably constituted, wholly or in part, by mutual inductance between the two indu'ctances LI, L2. In accordance with this invention there is connected across each tuning condenser CI, C2, a fixed resistance RCI, RC2 in series with a small variable control condenser VCII, VCZ and these ltwo control -condensers VC I VGZ are ganged with `the tuning condensers CII, CZ, and are of such law of variation that the inductance-resistance raitio for the circuits of which they form parts remains substantially constant over the whole tuning range of the filter. Although, for .the `sal-re of simplicity `the resistances Ril, R2, are shown as separate resistance elements, in practice they will usually be Aconstituted by the inherent resistances of the inductances LI, L2. The small variable control condensers VCI, VC2, are 'preferably 'constituted by a moving vane and `a fixed vane additional to each of the main tuning condensers. The additional fixed vane is insulated from the main fixed vanes but the movT ing vane need not be insulated and can be of the same shape as the main condenser moving vanes, The additional fixed vane is connected to the main fixed vanes through a suitable fixed resistance which thus in effect constitutes the resistance RCI or RC2 as the case may be and appears in effect as an addition to the main circuit resistance. The proportion of the total resistance which this additional part forms is dependent upon ithe size of the capacity between the auxiliary vanes (i. e upon the capacity VCI or VC2) and is itherefore varied as the filter is tuned. More specifically the additional resistance which is thrown into the tuned circuit, by means of the auxiliary end vanes and the iixed resistance, `is proportional to the square of the ratio of the auxiliary capacity Ito the main tuning capacity.

It is desirable that no resistance should be added at the highest frequency in the range, that is at the minimum setting of the tuning condenser. Also every'eiort should be made to make the main tuning capacity low at this point, so that in order to obtain a low ratio of auxiliary capacity to main capacity, the auxiliary capacity must be very small indeed. The xed minimum of the auxiliary capacity which occurs in practice, sets a limit to the ratio of the capacities and this ratio is sufficient to introduce more resistance into the tuned circuit than is desirable at the minimum tuning condenser setting. It becomes desirable therefore to supplement the fixed resistance by a suitably designed shunt impedance such that the effective resistance is reduced at the highest frequencies. One suitable form of shunt impedance comprises a simple series circuit permanently tuned to the highest frequency though, of course, other types of shunting impedance could be employed for the same purpose. Figure 2 shows an embodiment like Figure 1 except that series tuned circuits TCI, TG2 tuned, as just described, are connected across the resistances RCI, RC2 while, in the said Figure 2, the capacities VCI, VC2 are diagrammatically represented as constituted, as above described, by auxiliary vanes provided in the tuning condensers. To facilitate adjustment the auxiliary moving vanes may be segmented in the same way as that in which the normally provided end vanes of a present day ganged condenser unit are segmented for the purposes of adjusting the individual condenser sections.

The provision of the additional vanes does not in any way introduce difculty as regards ganging oi' condensers, for the usual segmented end vanes may be provided at the other ends of the individual condensers for this purpose. Thus, in the filter above described, the two tuning condensers could be constituted by two condenser sections in a ganged condenser unit having a single control shaft, each section having an adjustable segmented vane at each end. Two of these end vanes would be the normally provided ganging adjustment vanes and the other two would be the additional vanes provided by this invention.

Another and preferred way of overcoming any difliculty which may be experienced due to an undesirable high minimum capacity to earth of the auxiliary vanes consists in placing the said vanes,i11 each case, between two of the fixed vanes of the appropriate main tuning condenser. For example a thin layer of insulating material may be stuck to the face of a fixed vane of the appropriate main condenser and a layer of metal foilthis layer constitutes the auxiliary vane-stuck onto the insulating layer. Such an arrangement is illustrated in schematic section in Figure 3. Here FCI represents xed vanes of the tuning condenser CI and MCI moving vanes thereof. On one of the iixed vanes FCI is stuck a layer IL of insulation and on this is stuck a layer of metal foil FVCI which thus constitutes the auxiliary vane for the condenser VCI and is positioned between two of the vanes FCI. The resistance RCI is connected as shown between FCI and FVCI, the vanes MCI being earthed. The equivalent electrical circuit is shown in Figure 4 in which K represents the capacity of the resistance RCI and of the auxiliary electrode FVCI with respect to the fixed vanes FCI. As will be appreciated, the resistance RCI will have no effect unless there is capa-city from the foil FVCI to earth so that when the vanes MCI are moved out of interleaved position between vanes FCI, the resistance RCI is of substantially no effect. The capacity between FVCI and FCI merely shunts RCI. By suitably disposing or even shielding RCI its capacity may be also substantially wholly related to the fixed vanes' FCI instead of to earth. The position of the moving vanes MCI thus con trols the amount of additional resistance thrown into the circuit and by suitably shaping and disposing FVCI a desired law of resistance variation can be obtained. A similar arrangement is, of course, employed for C2, VCZ and RC2,

The main advantages obtainable by means of the invention are:

(l) Substantially constant performance over the tuning range;

(2) No interference with the coils of the iilter which can thus be designed to be of high Q value;

(3) No appreciable extra loss introduced at the high frequency end of the tuning range;

(4) No interference with ganging adjustments;

(5) No sliding contacts;

(6) Mechanical simplicity and cheapness of construction.

The arrangement of Figures 3 and 4 offers the advantages of extreme cheapness, ease of manipulation, avoidance of any necessity for fundamental mechanical modiiication of the tuning condensers, and maintenance of low minimum capacities for the tuning condensers.

The invention is not limited to the particular form of band pass lter described, but may be applied to other and more complex forms of lter networks, if required.

We claim:

1. A variably tunable band pass selection lter of the type comprising a plurality of coupled .resonant circuits, characterised by the use of mixed coupling between said circuits and the provision of means, uni-controlled with the variable tuning reactances of the lter, for varying the total effective resistance in each resonant circuit as the lter tuning is varied so that the ratio of inductance to resistance in each circuit varies according to a predetermined law with variation in tuning, said variation in resistance being obtained by providing, in association with each resonant circuit, a xed resistance in series with a small variable control condenser of predetermined law of variation, this condenser being gang controlled with the main tuning reactances of the lter.

2. A lter as claimed in claim 1 wherein the main tuning reactances are condensers and the control condensers are in the form of additional vanes` on the main tuning condensers.

3. A lter as claimed in claim 1 wherein the inain tuning reactances are condensers and the control condensers are constituted by end vanes of the main tuning` condensers.

4. A lter as claimed in claim 1 wherein the main tuning reactances are condensers and the control condensers are constituted by end vanes of the main tuning condensers and are positioned between iixed vanes of said tuning condensers.

5. A filter as claimed in claim 1 wherein the main tuning reactances are condensers and the contro1 condensers are constituted by end vanes of the main tuning condensers and are positioned between ixed vanes of said tuning condensers, the additional vanes being in the form of metal layers attached to insulating layers in turn attached to the faces of iixed vanes of the main tuning condensers.

6. A band-pass iiiter comprising a pair of similar tunable circuits, each comprising an inductance and a variable condenser, a series path connected in shunt across each tunable circuit, said path comprising a resistance and a small adjustable condenser, and means for simultaneously adjusting the tuning condensers and the small adjustable condens'ers for varying the total effective resistance in each tunable circuit, such that the ratio of inductance to resistance remains substantially constant for obtaining substantially constant band width and selectivity.

'7. A band pass lter as defined in claim 6 wherein the resistance of the series path is shunted by a series tuned circuit tuned to the highest frequency to which the tunable circuits are capable of being tuned.

ROBERT FRANCIS ONEILL. JOSEPH DOUGLAS BRAILSFORD. 

